A number of processes utilise gas dissolved in a liquid substrate. In order to maximise the dissolution of gas into the liquid, the gas bubble surface area should be maximised. This can be achieved by minimising the bubble size.
There are known methods and apparatus for producing these “microbubbles”, such as those described in U.S. Pat. No. 4,938,865 and AU677542. The apparatus described in these documents is known as the Jameson cell and facilitates the introduction of gas to a liquid stream to produce a foam layer. The Jameson Cell employs a single plunging jet of liquid to entrain atmospheric air via the Bernoulli effect which breaks it into very small bubbles within a zone of very high shear stress as the jet enters the liquid.
In the Jameson cell, the gas has to be injected at the top of a column and it is entrained by the high speed liquid. To enable gas entrainment to happen, the Jameson cell has a requirement of a minimum jet velocity. This is variously referred to as being 8 m/s or 15 m/s. It would be an advantage to be able to obtain microbubbles at a lower minimum jet velocity to increase energy efficiency of the system and provide increased flexibility by allowing jet velocity to vary based on the requirements of a specific application.
The mechanism of gas entrainment in the Jameson cell requires that the gas inlet orifice and the liquid jet have to be surrounded by a downcomer to generate a suction effect. The Jameson cell also requires a vessel additional to the pipe to receive the liquid jet and the mixture. The Jameson cell is designed for use in the froth flotation of minerals, and specifically promotes small particle attachment in the zone of high shear which causes high rates of viscous dissipation to heat. This requires high turbulence for better contact between mineral particles and gas bubbles. Because of this, the Jameson cell is characterised by its high turbulence in the downcomer. The high turbulence reduces efficiency of the overall system and may harm cells or proteins when used for particular applications such as fermentation by microorganisms.
Fermentation reactions using microorganisms are fed essential substrates in a gaseous form. For example gas streams containing CO and/or CO2 and/or O2 and/or H2 may be pumped into a bioreactor such that they bubble through the fermentation broth and/or may be provided in any headspace in the bioreactor. A portion of the gases in the streams dissolves in the fermentation broth such that it is then usable by the microbes active in the particular reaction. The availability or concentration of the gases in the fermentation broth can have a significant impact on the productivity of fermentation processes. However, gases such as CO and O2 have poor solubility in the generally aqueous broth contained within bioreactors, making it difficult and/or slow to dissolve desired quantities of the gases into the broth for use by the microorganisms in the fermentation process.
A potential method to enhance efficiency of gas fermentations by increasing gas-to-liquid mass-transfer is to sparge with microbubble dispersions. Such an enhancement has been demonstrated for a synthesis-gas fermentation involving Butyribacterium methylotrophicum grown in a continuous, stirred-tank reactor using a tangential filter for total cell recycle (Bredwell and Worden 1998, Biotechnol. Prog. 14, 31-38).
It is an object of the invention to overcome one or more of the disadvantages of the prior art, or at least to provide the public with a useful choice.